If you can run electricity out to the trough, a submersible trough heater works well. You have to protect the cord, naturally, so the animals don't have any way to gnaw on it. We found putting the trough halfway under a fence with the heater on one side and animals drinking on the other side worked well. If it doesn't go below zero much, a 250 watt heater will heat that 100 gallon trough pretty well. You can buy heaters in different sizes.
Of course if you're OTG you don't want to waste electricity to heat water…